This invention relates generally to perforating a web in the cross machine direction, and more particularly concerns dual perforation of scrim-reinforced webs to assure that all scrim filaments running in the machine direction are severed during perforation.
Nonwoven fabrics, such as melt-blown fabrics and spun-bonded fabrics, are frequently used as wipers in medical, commercial, and household applications. Such nonwoven material, while having excellent characteristics as wipers in terms of absorbency, linting, handling characteristics, and cost, frequently lack requisite tensile strength. In order to overcome the tensile strength deficiencies of nonwoven fabrics, the nonwoven fabric is sometimes laminated on either side of a scrim material which serves to reinforce the fabric. Typically, scrim consists of an open mesh of continuous filaments or yarns running both in the machine direction (length) and the cross machine direction (width) of the web. The filaments or yarns may be of any suitable material that is compatible with the nonwoven material to which the scrim is laminated. The continuous filaments of the scrim material impart the requisite tensile strength to the nonwoven fabric both in the machine direction and the cross machine direction.
Wipers or other sheet products manufactured from scrim-reinforced material typically are dispensed either from a roll or a box with a top opening. The individual sheets on a roll are torn from the roll one at a time along perforation lines. Alternatively, the individual sheets may be packed in a box in interfolded fashion. The interfolded individual sheets are dispensed from the top opening one at a time in the manner of the well-known Kleenex brand facial tissues. When the sheets are interfolded into a box, it is still necessary for the sheets to be attached to each other along a line of perforation so that as one sheet is pulled from the box, it will pull the next sheet into position to be subsequently dispensed from the box as it separates from the next sheet.
In either case, whether the sheets are dispensed from a roll by tearing one sheet directly from the next or from an interfolded stack in a box, it is still necessary to perforate the web to assure the proper tearing characteristics along the line of perforation.
Conventionally, perforation of a web has been accomplished by a single knife having notches at regular intervals along its length. Consequently, the knife produces a discontinuous cut with the notches creating uncut bonding points or portions along the width of the web that hold the individual sheets together until it is desired that the individual sheets be torn apart. By varying the width of the notches, and therefore the width of unsevered material or bonding points, the amount of tear strength at the perforation can be adjusted to provide suitable separation characteristics for the individual sheets made from a particular fabric.
In the case of scrim-reinforced material, the conventional single knife with notches may not provide a perforation which will be suitable. For example, if one of the machine direction filaments or yarns of the scrim falls within the width of the notch of the knife, that filament or yarn will not be severed and will provide a very strong bond point between the individual sheets which not only will remarkably change the tearing characteristics between the two sheets, but in the extreme may make it virtually impossible to separate the individual sheets without damaging the sheets at other points where the sheet is being grasped and pulled.
The problem of unsevered machine direction filaments or yarns in scrim-reinforced webs is recognized in Lewyckyj patents Nos. 3,716,132 and 3,835,754. Particularly, the Lewyckyj patents note that the uncut threads or filaments in the machine direction are an "impediment to separation along the lines of perforations [and] may result in a tearing of the body of the sheet to be separated." In addition, the unsevered machine direction filaments may result in an "application of a force to separate a sheet from the main roll [which] may result in an unwinding of the roll rather than the desired separation." (Lewyckyj patent No. 3,716,132, col. 1, lines 50-55.) In order to overcome that problem, Lewyckyj proposes applying a crushing force to the laminated product in the region of the reinforcing filaments which will substantially reduce the tensile strength of the reinforcing threads without substantially affecting the integrity of the cellulosic wadding which is laminated to the scrim-reinforcement. The crushing force is applied to the laminate structure prior to the perforation.